1. Field of the Invention:
The present invention relates to a method of forming a film of tantalum oxide on a substrate by plasma chemical vapor deposition, and more particularly to a method of forming a thin film of tantalum oxide having a high dielectric constant on a semiconductor substrate.
2. Description of the Related Art:
The fabrication of DRAMs (dynamic random access memories) having a storage capacity of 64 megabits (67, 108, 864 bits) or more requires that a film of tantalum (V) oxide (Ta.sub.2 O.sub.5) be formed as an insulation film of a high dielectric constant in the capacitance region of each memory cell of the DRAM. Heretofore, sputtering and chemical vapor deposition (CVD) are known to form a film of tantalum oxide on a substrate.
One example of a process of forming a film of tantalum oxide by CVD is a thermal CVD process using tantalum pentaethoxide and oxygen as reported by H. Shinriki et al. on page 25 of "1989 SYMPOSIUM ON VLSI TECHNOLOGY DIGEST OF TECHNICAL PAPERS" issued by the Electron Device Society of IEEE. A plasma CVD process which employs tantalum (V) chloride (TaCl.sub.5) and dinitrogen monoxide (N.sub.2 O) is reported by Y. Numasawa et al. on page 43 of "INTERNATIONAL ELECTRON DEVICES MEETING 1989 TECHNICAL DIGEST" issued by the Electron Device Society of IEEE.
Of the above processes for forming films of tantalum (V) chloride and dinitrogen monoxide is capable of forming films of better characteristics for use in the capacitance regions of memory cells. One conventional method of forming a film of tantalum oxide according to the plasma CVD process employing tantalum (V) chloride and dinitrogen monoxide will be described below with reference to FIG. 1 of the accompanying drawings. The plasma CVD system shown in FIG. 1 generates a plasma with high frequencies, and is known to a person of ordinary skill in the art.
As shown in FIG. 1, a certain quantity of tantalum (V) chloride (TaCl.sub.5), which is solid at room temperature, is placed in a cylinder 108, and heated therein by a temperature regulator 107, generating a vapor of tantalum (V) chloride. The generated vapor of tantalum (V) chloride is introduced with a carrier gas of argon through a filter 106 and a pipe into a plasma reaction chamber 101. The pipe is equipped with a pipe heater 105 which prevents the vapor of tantalum (V) chloride from being condensed.
The plasma reaction chamber 101 is evacuated by an evacuating system 110. The plasma reaction chamber 101 houses a susceptor 103 and a plasma electrode 102 that are disposed in confronting relationship to each other. The susceptor 103 serves to hold a silicon substrate 111 on which a film of tantalum oxide is to be deposited. The plasma electrode 102 is electrically connected to a high-frequency generator 104. The high-frequency generator 104 applies a high-frequency energy to the plasma electrode 102 to generate a plasma between the plasma electrode 102 and the susceptor 103, thereby forming a film of tantalum oxide on the silicon substrate 111.
The film of tantalum oxide thus formed on the silicon substrate 111 according to the above plasma CVD process has an electric capacitance of less than 10 fF per 1 um.sup.2. If the film were used in the capacitance region of a memory cell, it would not be able to reduce the area of the capacitance region. One reason is that oxidizing plasma sputtering progresses at the same time as the plasma CVD reaction particularly in the initial stage of the formation of a film of tantalum oxide. As a result, when a film of tantalum oxide is formed on a silicon substrate, the surface of the silicon substrate is simultaneously oxidized, forming a film of silicon oxide which has a thickness in the range of from 1 to 3 nm. As silicon oxide has a lower dielectric constant than tantalum oxide, the formed film for use as a capacitance film is composed of the film of silicon oxide and the film of tantalum oxide, thereby virtually reducing the capacitance per unit area.